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When the Gang of four introduced the singleton pattern, they also had to explain, why not to use static class fields and method instead. The reason was: the possibility to inherit. For Java it had sense - we cannot normally inherit the class fields and methods.
Later the "Effective Java" book appeared. And we know now that the existence of reflection destroys the singularity of the singleton class with private constructor. And the only way to make a real SINGLEton is to make it as a single item of an enumeration. Nice. I had done some myself this way.
But a question remains: While we cannot inherit from enumeration, what is the use of this singleton? Why we don't use these old good static/class fields and methods?
Edit. Thanks to the @bayou.io I see that in https://softwareengineering.stackexchange.com/a/204181/44104 there is a code that can trick the enum, too, and create again two exemplars of the enum singleton. The other problems are mentioned there, too. So, there is no need to use enum instead of the usual singleton class pattern, too? BTW, all enum pluses that are mentioned here till now, work for singleton classes, too.
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As stated in point 1 since creation of Enum instance is thread-safe by default you don't need to worry about double-checked locking. In summary, given the Serialization and thread-safety guaranteed and with a couple of lines of code enum, the Singleton pattern is the best way to create Singleton in Java 5 world.
The Singleton's purpose is to control object creation, limiting the number of objects to only one. Since there is only one Singleton instance, any instance fields of a Singleton will occur only once per class, just like static fields.
The main objective of enum is to define our own data types(Enumerated Data Types). Declaration of enum in Java: Enum declaration can be done outside a Class or inside a Class but not inside a Method.
The advantage of Singleton over global variables is that you are absolutely sure of the number of instances when you use Singleton, and, you can change your mind and manage any number of instances.
what is the use of this singleton? Why we don't use these old good static/class fields and methods?
Because enum is an object so it can not only be passed around but also implement interfaces.
Also since we are making a class, we can use the different public/private options available to all kinds of classes.
So in practice, we can make a singleton that implements an interface and then pass it around in our code and the calling code is non the wiser. We can also make the enum class package private but still pass it around to other classes in other packages that expect the interface.
If we used the static methods version, then the calling class would have to know that this object is a singleton, and our singleton class would have to be public so the other classes can see it and use it's methods.
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There's nothing particularly wrong with the "good old fashioned singleton", enum "singletons" are just convenient - it saves you the need to muck around with boiler-plated code that looks the same in every singelton.
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